A two-component developing method using a mixture of an insulating nonmagnetic toner and magnetic carrier particles has heretofore been known useful for the system of developing electrostatic latent images in electrophotography. In the two-component developing method, the carrier plays the role of frictionally charging the toner and carrying it onto the surface of a photoreceptor on which the charged toner is contacted with electrostatic latent images.
Of the particulate carrier to be used in such two-component development, the magnetic carrier core is generally coated with a suitable resin material for purposes of ensuring the spent resistance of the carrier (for preventing spent toner from filming on the surfaces of carrier particles), making carrier particles have a uniform surface, prolonging the life of developers comprising the carrier, preventing the surface oxidation of the carrier, improving the weather resistance of the carrier, protecting photoreceptors from the carrier in order not to be scratched or worn by the carrier, controlling the resistance of the carrier, controlling the chargeable polarity of the carrier, and controlling the degree of charging of the carrier.
As such coated carriers, for example, known are polyolefinic resin-coated carriers (see, for example, Japanese Patent Application Laid-Open Nos. Sho-52-154639 and Sho-54-35735).
Precisely, Japanese Patent Application Laid-Open No. Sho-52-154639 discloses a polypropylene resin-coated carrier, which is produced by heating and melting a polypropylene resin in a suitable solvent followed by spraying the resin melt onto a carrier core. Japanese Patent Application Laid-Open No. Sho-54-35735 discloses a coated carrier, which is produced by applying a powder of a coating material onto carrier particles followed by heating the coated carrier particles at a temperature not lower than the melting point of the coating material to thereby fix the coat on the carrier particles.
However, those polyolefinic resin-coated carriers are defective in that the adhesion between the coating layer and the carrier core is poor and therefore the durability of the carriers is poor. For example, when the carriers are used in continuous copying operations, the coating layer is often peeled off from the carrier core. In addition, the methods of producing those coated carriers are problematic in that the thickness of the coating layer is difficult to control.
In order to solve those problems, a technique has been proposed of processing the surface of a carrier core with an olefin polymerization catalyst followed by directly polymerizing an olefin on the thus-processed surface of the carrier core to produce a resin-coated carrier. For example, using this technique, produced are polyolefinic resin-coated carriers with good electrostatic characteristics, spend resistance, charge stability and weather resistance, of which the surface of the coating resin has a roughened structure (see, for example, Japanese Patent Application Laid-Open Nos. Hei-2-187770, Hei-2-187771 and Hei-3-208060).
The resin coats of those resin-coated carriers are effective in preventing the change in the physical properties of the carriers themselves during the use of the carriers and also the change in the physical properties of photoreceptors to which developers comprising the carriers are applied. However, the resin-coated carriers, as having such roughened surfaces, are not always satisfactory since their surface conditions often vary, while the carriers are used for a long period of time in developing machines, due to the shear of the carrier particles themselves being stirred and due to the shear of the carrier particles and toner particles being stirred together, and even due to the stress of those particles against doctor blades, resulting in that the carriers could not sufficiently keep their original physical properties such as electric resistance. As a result of long-term use of those resin-coated carriers, the quality of images formed is often worsened.
In addition, the resin-coated carriers are further problematic in that their resin coats are often partly cut off from their surfaces due to the shear of the carrier particles being stirred in developing machines and due to the repeated contact of the carrier particles with doctor blades in developing machines, resulting in that the resin thus cut away from the carrier particles adheres onto the doctor blades. The adhesion of the resin onto the doctor blades often causes insufficient supply of toners to the developing zone in the developing machines, thereby producing other problems in that the density of images formed is lowered, that streaks are formed in the images and that the quality of the images is worsened. For these reasons, the resin-coated carriers are not always satisfactory.
The doctor blade as referred to herein is meant to indicate a metal plate that acts to control the thickness of the carrier layer to be formed on the magnetic sleeve of a developing machine, and generally, it is made of brass, stainless steel or the like.
The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a carrier with good durability for electrophotography, of which the properties do not change in its long-term use so that the images formed using the carrier always have their original quality, and to provide a developer for electrophotography that comprises the carrier.
Specifically, the object of the invention is to provide a resin-coated carrier for electrophotography, of which the resin coat does neither peel off nor adhere onto the doctor blades in developing machines in its long-term use, and which is stably used for a long period of time without worsening the quality of images being formed, and to provide a developer for electrophotography that comprises the carrier.